Composite metal tube and method of making the same

ABSTRACT

A composite small-diameter tubing resistant to interaction with materials conveyed therethrough such as liquids containing alcohols or corrosive agents which includes a cylindrical outer metal conduit having a hollow central shaft extending longitudinally therethrough defined by an interior wall; and a continuous non-reactive interior tube surrounded by and positioned within the cylindrical metal conduit. The tubing can be manufactured by a process which includes the steps of a) positioning a length of non-reactive tubing having a predetermined outer diameter within an inner diameter of metal tubing of essentially corresponding length, the metal tube having an unsealed side seam, wherein the inner diameter is greater than the outer diameter of the non-reactive tubing; and b) sealing and reducing the metal tubing to an outer diameter essentially equal to the predetermined outer diameter of the interior tubing. The non-reactive interior tube may be made of a suitable polymeric material.

This application is a division of U.S. application Ser. No. 07/806,173,filed on Dec. 12, 1991, now U.S. Pat. No. 5,339,867.

BACKGROUND OF THE INVENTION

1. Field of the Invention

The present invention relates to small-diameter lined tubes which areresistant to interaction with chemicals conveyed therethrough and amethod for making the same. More particularly, this invention pertainsto composite tubing stock lined with a corrosion-resistant material anda method for successfully producing the same.

2. Discussion of the Relevant Art

Small-diameter tubing, tubing stock having a diameter less than about 1inch and more particularly between about 3/16 and about 3/4 inch, isused for a variety of applications. Among these are brake linings, fuellines, and various conduits in a variety of non-automotive uses. It isnecessary that these products be manufactured from durable materialswhich can withstand prolonged use as well as bumps, jostling and thelike. In automotive applications, metals such as low-carbon steel arethe materials of choice for reasons of economy and formability, forexample. In other applications, other metals or various polymericmaterials have been employed.

It is also necessary that the material of choice be essentiallynon-reactive with the materials conveyed through the tubing. Variousfluids conveyed through the tubing exhibit degrees of reactivity withinterior tubing surfaces. For instance, various alternate fuels havecomponents such as methanol (contained in fuels such as M85) and ethanol(in E85). These components can react with metals such as low carbonsteel to cause corrosion. The resulting corrosion can weaken the tubing;eventually resulting in rupture, leakage and premature failure.Additionally, even materials which are not directly corrosive in ametallic environment can render the metal susceptible to exteriorgalvanic corrosion. Thus, the useful life of conventional low carbonsteel is shortened even when conveying only mildly reactive materials.

Reactivity between the tubing and the materials conveyed within it canalso compromise the purity of the conveyed fluid materials. Because ofthis, inexpensive metal tubing cannot be used in a variety ofapplications where possible fluid contamination with metal complexes,liquids or the like are not acceptable. In such instances, it isnecessary to subject the tubing to post-formation processes such asannealing or to resort to more expensive tubing made of non-reactivemetals or to tubing made from polymeric materials.

Polymeric tubing has been suggested as a substitute in various instancesto eliminate the problems such as those previously discussed.Unfortunately, polymeric tubing presents a different set of problems.Conventional polymeric tubing is generally composed of materials such asvinyl, polypropylene, polyvinylchloride and the like. These polymersexhibit poor formability characteristics. Because the polymericmaterials exhibit elastic memory, the tubing constructed from suchmaterials is difficult to permanently contour.

Prolonged use of sections of polymeric tubing can lead to thedevelopment of static charge. In this phenomenon, static charge buildsup along the plastic line and, ultimately, results in numerous,unpredictable pinhole ruptures in the tubing. As can be appreciated, theuse of polymeric lines is not desirable in high pressure applications orin situations in which the tubing will convey flammable liquids whichcould ignite upon escape during rupture. Furthermore, it is difficult toachieve burst strength characteristics in monolithic structures even ifproblems of static discharge can be obviated.

Even if the problems of formability and static discharge can beovercome, polymeric tubing exhibits extreme weakness to heat. Thepolymeric materials employed in conventional plastic tubing sag, weakenor melt at undesirably low temperatures; rendering them impractical formany applications such as use in conjunction with automobile engines.

The materials employed in polymeric tubing also exhibit degradation overtime due to interaction with other external environmental factors suchas exposure to ultraviolet radiation which results in cross linkage, UVdegradation and the like. This reduces tubing flexibility, therebyrendering the tubing brittle and easily breakable.

Finally, polymeric materials which make up the tubing can interact withcertain organic components causing softening, localized deformation ofthe tubing, or permeation of the organic material through the polymericmaterial. Thus, a variety of organic fluids cannot be conveyed throughpolymeric tubing stock.

The use of composite or lined tubing stock composed of an outer metallayer and an interior non-reactive polymeric layer has been proposed.However, to date, only metal tubes having relatively wide innerdiameters have been produced by spraying a liquid polymeric materialonto the interior of the metal tube by means of an appropriate spraynozzle or other suitable dispensing device inserted into the interior ofthe metal tube to deliver molten polymeric material directly onto theinterior wall. This method generally limits the production of suchtubing to tubing stock having an inner diameter of sufficient width topermit insertion of the polymer dispensing device. Because the polymerapplicator can only be inserted a relatively short distance into themetal tube, the overall length of lined tubing produced by this methodis limited. Given these constraints, it can be appreciated that it hasbeen impossible to employ the method to small-diameter tubing.Furthermore, it has been difficult to assure that the applied polymerplastic is uniformly dispersed over the interior surface of the tubing.Non-uniformities of the polymer deposited on the interior surface of thetubing can result in unwanted narrowing of inner diameter of the tubingcausing constriction or blockage or insufficient polymer coating toprevent interaction with the reactive or corrosive fluids conveyedtherethrough.

Thus, it is desirable to provide a tubing which exhibits enhancedresistivity to interaction with fluids conveyed therethrough. It is alsodesirable that this tubing be formable and machineable in subsequentpost production operations. It is also desirable that this materialcontain a uniform non-reactive lining along its interior surface. It isdesirable that the tubing be of sufficiently narrow diameter to permitits use in automotive fuel lines and the like. Finally, it is desirableto provide an inexpensive effective method for producing such a materialwhich eliminates the need for post-treatment processing steps such asannealing.

SUMMARY OF THE INVENTION

The present invention is a composite tubing resistant to interactionwith materials conveyed therethrough and a method for making the same.The tubing may be either continuous lengths which can be coiled and cutto length as required or produced immediately as finished lengths. Thecomposite tube of the present invention comprises a cylindrical outermetal conduit having an outer wall and an interiorly oriented wall whichdefines a central shaft extending longitudinally therethrough, and aninterior tube made of a suitable non-reactive material surrounded by andpositioned within the cylindrical outer metal conduit. The interior tubehas outer and inner wall surfaces. The outer wall surface of theinterior tube matingly engages the interiorly oriented wall of the metalconduit by any suitable manner. The interior tubing conforms to theinteriorly oriented wall of the metal tubing to surround and define ahollow tubing conduit extending longitudinally through the compositetubing and to protect the surface of the interiorly oriented metal wallfrom contact with potentially reactive fluids conveyed therethrough.

The method of the present invention comprises the steps of:

positioning a length of tubing made of a suitable non-reactive materialhaving a predetermined outer diameter within the interior of an unsealedmetal tube having an initial inner diameter greater than thepredetermined outer diameter of the polymeric tubing; and

progressively reducing the metal tube after sealing to produce acomposite metal tubing having an outer diameter essentially equal to thepredetermined outer diameter of the inner non-reactive tubing containedtherein.

BRIEF DESCRIPTION OF THE DRAWINGS

In order to more fully understand the composite tube of the presentinvention and the method of making the same, the following drawingfigures are included in which like reference numerals are usedthroughout in connection with like elements and in which:

FIG. 1 is a schematic drawing of the composite tube forming process ofthe present invention; and

FIG. 2 is a detail drawing in cross-section taken at the welding stationalong the 2--2 line of FIG. 1 with the pressure roll broken away.

DESCRIPTION OF THE PREFERRED EMBODIMENT

The present invention is a composite tube which has an interior surfaceresistant to interaction or reaction with the materials which areconveyed therethrough. The reactive materials can include variousorganic fluids such as short-chain alcohols, petroleum fuels containingreactive additives as in the case of alternative fuels, as well asvarious materials known to be corrosive to metal tubing such as thoseemployed in desalinization units or the like. Other similar reactive andnon-reactive fluids can be successfully conveyed through composite metaltubing of the present invention.

The tubing of the present invention comprises a cylindrical outer metalconduit having an outer wall, an interiorly oriented wall which definesa central shaft extending longitudinally therethrough, and a continuousinterior tube made of a suitable non-reactive material surrounded by andpositioned within the cylindrical outer metal conduit. The continuousinterior tube has an outer wall surface and an inner wall surface. Theouter wall surface of the interior non-reactive tubing contacts theinteriorly oriented wall surface of the metal conduit to provide matingengagement between the two concentrically disposed tubes in a mannerwhich is essentially uniform throughout the length and circumference ofthe composite tubing.

The cylindrical outer metal conduit may be made from any metal or metalalloy or other metallic, organometallic or metal matrix compositematerial which can be suitably formed, bent, or machined for subsequentfluid conveying operations. These materials include but are not limitedto materials which are reactive with various chemicals or chemicalconstituents conveyed therethrough or are susceptible to galvanic orchemical corrosion. The outer metal conduit is preferably made of asuitably formable and weldable material, with a metal selected from thegroup consisting of ferritic metals, non-ferritic metals, and alloysthereof being preferred. In situations requiring tube ease offormability and economy, low carbon steel is preferred. Various gradesof low carbon steel can be successfully employed in the composite tubingof the present invention. Specifics of steel type vary depending uponend use application but could readily be discerned by one skilled in theart.

The cylindrical outer metal conduit may have a finished outer diameterof any size desired for the given end use application. The outerdiameter of the tubing of the present invention is preferably less than,but not restricted to about 3/4 inch with an outer diameter between 3/16inch and 3/4 inch being preferred. The wall thickness of the cylindricalouter metal conduit is dependant on the end use application for thecomposite tubing as well as the drawing and welding operations employedin the tube formation process. In general, the outer metal conduitcomprises up to about 99% of the total wall thickness of the compositetube of the present invention with wall thicknesses of the outer conduitbetween about 40 and about 99% being preferred. The thickness of thewalls of the outer metal conduit may be less than about 0.3 inch; withthicknesses between about 0.01 inch and about 0.1 inch being common andthicknesses between about 0.02 inch and about 0.05 inch being preferred.It is to be understood that the wall thickness of the outer metalconduit can be varied according to the requirements for the particularapplication of metal tubing.

The composite metal tube of the present invention also includes acontinuous interior tube made of a suitable non-reactive material havingan outer wall surface in contact with the interiorly oriented wall ofthe cylindrical outer metal conduit to provide essentially uniformmating engagement between the two concentrically disposed tubes. Theinterior tube conforms to the interiorly oriented wall of the outermetal conduit. The inner wall surface defines a hollow central tubingconduit extending longitudinally through the length of the tubing.

The interior tube may be constructed from any suitable material which isnon-reactive in the particular application for which the tubing is to beemployed. The material may be either metal or polymeric as desired andrequired.

The non-reactive material employed may be any material suitable forformation of lengths of interior tubing which is non-reactive withmaterials to be conveyed therein. The term "polymeric" as employedherein may be defined as a polymolecular complex formed by the union ofsimple organic monomers. The polymeric material employed herein may beany homopolymer, polymeric blend or copolymeric material which issubstantially non-reactive in the presence of the chemicals or compoundsto be conveyed through the tubing. In particular, the polymeric materialemployed is substantially non-reactive in the presence of short chainalcohols, various corrosive agents, metal oxidizers, and the like.

The polymeric material of choice is a thermoplastic material exhibitingthermal stability at temperatures of approximately 500° F. The materialemployed is preferably extrudable as either a monoaxially or biaxiallyoriented organic film. In the preferred embodiment, the material iscapable of extrusion to thicknesses less than about 0.02 inch withextrusions to thicknesses less than about 0.005 inch being preferred. Inthe preferred embodiment, the polymeric material is selected from thegroup consisting of polyamide resins, polytetrafluoroethylene resins,and mixtures thereof. Examples of suitable polyamide resins for use inthe present invention include the nylons. Examples of suitablepolytetrafluoroethylene resins include those having the tradedesignation TEFLON.

The non-reactive material employed in the composite tubing of thepresent invention has a wall thickness sufficient to isolate theinteriorly oriented wall surface of the outer metal conduit from anyharmful effects of any fluid conveyed therethrough. The thickness of thenon-reactive liner is sufficient to maintain the integrity of theinterior tubing without unduly compromising the heat exchange capabilityof the composite tubing.

The interior non-reactive tube is positioned within the cylindricalouter metal conduit so as to engage its interiorly oriented wall in anessentially uniform manner such that the central shaft is completelydefined by and surrounded by non-reactive material throughout itslongitudinal length.

The engagement between interior tubing and the outer metal conduit maybe any suitable mechanical, chemical or a mixture of mechanical andchemical attachment properties. The term "mechanical attachment" isdefined herein to encompass physical engagement between the twoconcentrically disposed tubes which includes but is not limited tointerference fits and other frictional and/or compressive engagements.The term "chemical attachment" is defined herein to include any chemicalor electrochemical bonding which may occur between the twoconcentrically disposed tubes. "Mixtures of mechanical and chemicalattachment properties" is defined herein to include attachment phenomenasuch as adhesion. In the preferred embodiment, the engagement betweenthe two concentrically disposed tubes is an interference fit.

The composite tubing of the present invention provides the corrosionresistance and resistance to chemical reactions heretofore not found insmall-diameter monolith metal tubing while eliminating the problemsfound in monolith plastic lines such as melting, static discharge, poorformability, pressure restriction, permeation, and degradationpreviously inherent in supplemental or polymeric tubing.

The steel tubing of the present invention can be formed by a process inwhich a length of interior tubing non-reactive material having apredetermined outer diameter is positioned within the inner diameter ofmetal tubing of a suitably analogous length. The predetermined outerdiameter of the non-reactive tubing preferably is essentially equal tothe desired outer diameter of the finished composite tubing. The metaltubing has an initial inner diameter greater than the outer diameter ofthe non-reactive tubing. Once the non-reactive interior tubing is inposition, the external metal tube can be reduced in size by suitableforming and drawing operations to an outer diameter essentially equal tothe predetermined outer diameter of the non-reactive interior tubing. Atthe outset of processing, the metal tubing, preferably, has an unsealedside seam extending along the length of the tube through which thepolymeric tubing can be inserted. The metal tubing is sealed afterinsertion of the polymeric tubing by any suitable sealing means such asan electrical resistance, TIG, MIG, or laser welding or even lock-seamclosure. Once sealed, the metal tube can be drawn and formed to reducethe tubing diameter until it snugly engages the polymeric tubingcontained therein to provide concentric orientation of the two tubingmaterials, one within the other.

For a more complete understanding of the method of making the compositetube of the present invention, attention is directed to the schematicdepiction of the method of the present invention as set forth in FIGS. 1and 2 and discussed as follows.

The cylindrical outer metal conduit may be previously formed andprovided from existing stock into which the non-reactive tubing can beinserted. In the preferred embodiment, the conduit is gradually formedfrom suitable flat sheet metal stock.

The flat sheet metal stock may be maintained in any manner which willpermit its uniform conveyance into and through various tube formingstations. As illustrated in FIG. 1, a continuous portion of flat sheetmetal stock 12 maintained on a suitable pay-out reel 14 is paid out fromthe reel 14 and drawn into tube forming machinery 10 where the sheetmetal stock 12 is oriented, sized and operated on by a plurality offorming rolls 16 to progressively form the flat sheet stock into acontinuous unsealed metal tube.

As can be appreciated by one skilled in the art, in the initial formingoperations the sheet metal stock 12 passes in contact with convexrollers 16A which form the interior and exterior of the tubing surface.The sheet metal stock 12 then comes into contact with concave rollers16B which operate only on the exterior surface of the tubing beingformed to bring the two side edges into abutment. The exterior sectionsbring the two longitudinal edges of the sheet metal stock 12 intosufficiently close proximity or abutment with one another to permitsubsequent sealing along the seam thus formed.

The interior non-reactive tubing to be employed, i.e. polymeric tubing,may be brought into contact with the metal sheet stock 12 at any timebefore, during or after the initial roller forming operations. In thepreferred embodiment, continuous polymeric tubing 18 is introduced intorelationship with the metal sheet stock 12 during roller formingoperations.

The continuous polymeric tubing 18 is preferably positioned in the metaltubing in formation during or immediately subsequent to theinterior/exterior formation passes implemented by convex rollers 16A. Itis preferred that the continuous polymeric tubing 18 enter the tubing ata point in the interior/exterior forming process where enough curvaturehas been imparted to the sheet metal stock 12 to assist in maintainingthe polymeric tubing 18 in position relative thereto. In the preferredembodiment the polymeric tubing is brought into contact with the sheetmetal stock 12 immediately prior to the commencement of external formingoperations performed as the sheet metal stock passes through concaverollers 16B.

In the process of the present invention, the metal sheet stock 12 ispaid off of the payoff 14 and fed to the forming rollers 16 at a firstrate (R₁). The first rate R₁ is sufficient to permit appropriateformation of the sheet metal stock into unsealed tubing as well aspermitting sealing of the tubing stock in subsequent steps. While thefirst rate R₁ can be varied, it is generally desirable to maintain afirst rate R₁ as high as possible to maximize tubing production. Forthis reason, feed rates between about 150 and about 180 feet per minuteare preferred but speeds up to 250-450 feet per minute can beenvisioned.

In the process of the present invention, the polymeric tubing ispreferably introduced into the metal tubing at any point prior to thewelding step. In the preferred embodiment, the polymeric tubing isintroduced into the metal tubing immediately prior to external metaltube forming operations performed by external rollers 16B. The rate ofintroduction or payout of the polymeric tube 18 into the metal tube isgreater than the payout rate of the sheet metal stock that forms thesurrounding outer metal tube. The differential in payout rates causesthe polymeric tube to more rapidly pass through the tube sealing zoneand to be maintained in an appropriately taut manner through finalforming and drawing steps.

In metal tube forming processes such as that of the present invention,the sealed metal tubing is subjected to post-sealing sealing sizing anddrawing operations during which the outer diameter of the sealed metaltube is reduced with concurrent elongation of the tubing stock. Thus,the finished product passes from the final forming station 28 at a rate(R₂) faster than the payout rate of the unformed sheet metal stock 12from reel 14. The increase in speed is proportionally related to thedecrease in diameter in the finished product.

In the preferred embodiment of the process of the present invention, thepolymeric tube 18 is paid out into the partially formed metal tubing ata rate approximately equal to the production rate of the finishedcomposite tube.

The speed differential between the polymeric tube 18 and the outer metalconduit upon introduction of the tubing is a function of the diameterreduction of the outer metal tubing to be accomplished during the finalsizing and forming operations. Preferably, the outer diameter of theouter metal tubing is reduced to a diameter approximately equal to orslightly less than the outer diameter of the polymeric tubing containedtherein by progressive contact with sizing rollers 28A and 28B whichwill be described in detail subsequently. In the preferred embodimentthe polymeric tubing 18 is introduced at a rate equal to the ratefinished composite tubing passes from the final forming station 28.

The polymeric tubing employed in the process of the present inventionhas an outer diameter which is approximately 30 to about 70% of theouter diameter of the unsealed metal tubing formed by rollers 16. Thepayout differential between the two materials is approximately equal tothis diameter differential. The diameter of the polymeric tubing 18 isless than about 1 inch with diameters between about 3/16 inch and about3/4 inch being preferred.

The polymeric tubing 18 employed herein may have any suitable wallthickness. The wall thickness may be limited by the tensile strength ofthe chosen polymeric material during tube formation. In the preferredembodiment, polymeric wall thicknesses between about 0.005 inch andabout 0.01 inch are employed.

The polymeric tubing may be made of any suitable thermally stablethermoplastic material. Materials having thermal stability ofapproximately 500° F. are preferred. The thermoplastic material employedexhibits suitable tensile strength and is unreactive with a variety ofchemical materials. As indicated previously, the polymeric tubingpreferably consists of a polymer selected from the group consisting ofpolyamide resins, polytetrafluoroethylene resins, and mixtures thereof.

The polymeric tubing 18 may be produced by any process which provides anadequate supply of tubing stock for the formation process of the presentinvention. The polymeric tubing 18 may be extruded concurrent with thetube forming process. In such conformation processes, the sheet metalstock payout rate would be modified to provide a final tube productionrate compatible with the tubing extrusion rate. Alternately, thepolymeric tubing formed in a separate process can be coiled onto aseparate polymeric tubing payout reel 30 to permit flexibility inadjusting the rate of tube formation.

The polymeric tubing 18 is paid out into the partially formed metaltubing at a point adjacent to but immediately upstream of gas injectionpipe 22. As shown in FIG. 1, gas injection pipe 22 is inserted into thepartially formed metal tubing immediately before the first externalforming operation at roller 16B. The gas injection pipe 22 extendsthrough the interior of the metal tubing being formed and terminatesproximate to and slightly downstream of the tube sealing means 20 toprovide cooling and oxidization protection of the sealed metal tubing bydirecting an inert gas such as nitrogen at the newly sealed seam.

The gas injection pipe 22, preferably, has a cross-sectional contourcapable of directing and orienting the polymeric tubing 18 in positionin the outer metal tubing, the gas injection pipe 22 preferably has aconcave divot 23 extending longitudinally along the pipe 22 which cancontact the polymeric tubing passing thereby. The gas injection pipe 22is interposed between the polymeric tubing 18 and the side edges of thesheet metal stock 12 being brought into abutment through the process ofthe present invention. This insures proper radial positioning of theinner tube during sealing procedures. The non-reactive polymeric tubing18 is maintained at a location within the metal tubing 12 opposed to theside seam during the sealing step, as best seen in FIG. 2.

After the tube formation stage, the unsealed metal tubing with thepolymeric tubing 18 contained therein is sealed by any suitableprocedure. The sealing procedure preferably provides a sealed seamhaving a narrow width; i.e. between about 0.01 and about 0.1 inch. Thenarrow seam seal occurs in a manner such that any given region of theseam and adjacent metal tube area remains at temperature for an intervalless than about 0.5 to 1.5 seconds. Rapid heat dissipation can beaugmented further by post sealing gas cooling, water cooling or acombination of both if desired.

Sealing is preferably effected by localized welding. In the preferredembodiment, electric resistance welding is employed. As shown in FIG. 2the unsealed seam is brought into contact with suitable weldingelectrodes 24, 24' held in place by an appropriate pressure roll 26. Inthe preferred embodiment, a 20° arc is employed. The tubing passes bythe welding source 20 at a rate capable of providing a suitable uniformweld W.

The sealed metal tubing can then be subjected to post welding steps suchas exterior weld metal scarfing and integrity assurance testing (notshown). Once sealed, the sealed outer metal tubing with the smallerdiameter polymeric tubing contained inside can pass to the drawing andforming section 28 where diameter reduction of the outer metal conduitcan be accomplished. The drawing and forming section 28 consists of aplurality of powered reducing rollers 28A and idle rollers 28B whichboth convey the outer metal tubing onward and progressively reduce theouter diameter of the sealed metal tubing. The amount of diameterreduction is that sufficient to achieve an interference fit between theouter metal tube and the polymer tube contained inside. In the preferredembodiment, diameter reduction up to and including 50% withcomplementary increases in the payout rate of the formed, sealedreduced-diameter metal tube are obtained.

While the exterior metal tubing is conveyed by powered reducing rollers28A, in the preferred embodiment, the polymeric tubing is drawn by itsleading edge which is in interfering engagement with the outer metaltube. Thus the polymeric tubing is drawn into the metal tube beingformed at a rate essentially equal to the finish rate of the sealedmetal tubing (R₂). The polymeric tubing payout means is appropriatelyconfigured to provide suitable tension throughout the polymeric tubing.

In the method of the present invention, the opportunity for thermaldegradation of the polymeric tubing is greatly reduced because of theaccelerated feed rate of the polymeric tubing past the weld source.Additionally, the radial positioning of the polymeric tube away from theheat source prevents damage during the sealing process. Finally, theinterposition of the gas injection pipe between the polymeric tubing andthe weld spot further insulates and protects the polymeric tubing fromthermal degradation.

Once the outer tubing has been roller reduced and the interference fitbetween the polymeric tubing and the outer metal tubing has beenobtained, the composite tubing may be cut to length, formed, machined orcoiled and stored as required. The need for subsequent tube treatmentprocesses such an annealing is eliminated.

The process outlined may also be employed using other non-reactive innertubes. Alternately, the composite tubing of the present invention may beproduced by a draw die method in which the inner non-reactive tubing ofthe approximate desired finished diameter is inserted into a preformedouter tubing having an inner diameter greater than the outer diameter ofthe inner tube. The two distinct tubes are pulled from the output end ofthe tube forming mechanism. The diameter of the outer tube isprogressively reduced to mating contact with the inner tube by passagethrough suitable die sets.

We claim:
 1. A method for manufacturing a metal tube having an interiorsurface resistant to interaction with materials to be conveyedtherethrough, the method comprising:positioning a length of non-reactivetubing having a predetermined outer diameter within an inner diameter ofmetal tubing of essentially corresponding length, said metal tube havingan unsealed side seam, wherein said inner diameter of said metal tube isgreater than said outer diameter of said polymeric tubing; sealing saidunsealed side seam of said metal tube while maintaining saidnon-reactive tubing at a location within said metal tubing opposed tosaid side seam during said sealing step; and reducing said sealed metaltubing to an outer diameter essentially equal to said predeterminedouter diameter of said non-reactive interior tubing.
 2. The method ofclaim 1 wherein said inner diameter of said unsealed metal tubing is atleast 30% greater than said outer diameter of said non-reactive tubing.3. The method of claim 1 further comprising the step of:introducing astream of gaseous material into said metal tubing at a localized areaimmediately adjacent to said side seam, said stream of sufficient volumeto transfer heat generated in said sealing step away from said seam in adissipative manner, said introduced stream passing between said seam andsaid non-reactive tubing.
 4. The method of claim 2 wherein said metaltubing having a first outer diameter D₁ is sealed at a first rate R₁,and said reduced metal tubing having a final outer diameter D₂ less thansaid first diameter D₁ is produced at a second rate R₂, said second rateR₂ greater than said first rate R₁ wherein diameter reduction isproportional to rate acceleration;and wherein said non-reactive tubinghaving an outer diameter essentially equal to said final outer diameterD₂ is introduced into said unsealed metal tubing and travels past meansfor implementing said sealing step at a rate approximately equal to saidsecond rate R₂, said unsealed tubing traveling at a rate essentiallyequal to said first rate R₁ past said sealing means.
 5. The method ofclaim 2 wherein said non-reactive tubing is maintained in tensionrelative to said metal tubing during said sealing step, said tensionbeing sufficient to orient said non-reactive tubing relative to saidmetal tubing at a position which prevents contact between said side seamand said non-reactive tubing during said sealing step.
 6. The method ofclaim 2 wherein said reducing step comprises successive exposure of saidsealed metal tubing to progressively narrower roller forming units. 7.The method of claim 1 wherein said non-reactive tubing has an outerdiameter less than about 3/4 inch.
 8. The method of claim 5 wherein saidnon-reactive inner tubing is a flexible polymeric material which isessentially non-reactive when brought into contact with materials whichare interactive with metal surfaces.
 9. The method of claim 5 whereinsaid non-reactive inner tubing is extruded in a continuous manner intosaid unsealed metal tube through said unsealed side seam.
 10. The methodof claim 8 wherein said polymeric tubing material consists essentiallyof a polymer selected from the group consisting of polyamide resins,polytetrafluoroethylene resins, and mixtures thereof.
 11. The method ofclaim 7 wherein said interior tubing has a wall thickness less thanabout 0.005 inch.
 12. A method for manufacturing a lined metal tubecomprising the steps of:inserting a length of polymeric tubing having apredetermined outer diameter into an inner diameter of metal tubing ofessentially corresponding length, said metal tube having an unsealedside seam and an inner diameter at least 30% greater than saidpredetermined outer diameter of said polymeric tubing, said metal tubetraveling at a first rate and said polymeric tubing being inserted at asecond rate greater than said first rate yielding a rate differentialessentially equal to a differential of said diameters; welding saidunsealed side seam while maintaining said polymeric tubing at a locationwithin said metal tubing opposed to said side seam during said welding;and reducing said metal tubing to an outer diameter essentially equal tosaid predetermined outer diameter of said polymeric tubing.
 13. Themethod of claim 12 wherein said polymeric tubing is maintained intension relative to said metal tubing during said sealing step, saidtension being sufficient to orient said polymeric tubing materialrelative to said metal tubing.
 14. The method of claim 12 furthercomprising the step of:machining said resulting weld surface to conformto said outside surface of said tubing.
 15. The method of claim 13wherein said reducing step comprises successive exposure of said sealedmetal tubing to progressively narrower roller forming units.
 16. Themethod of claim 13 wherein said polymeric tubing has an outer diameterless than about 3/4 inch.
 17. The method of claim 16 wherein saidpolymeric tubing is extruded in a continuous manner into said unsealedmetal tube through said unsealed side seam.
 18. The method of claim 14wherein said polymeric tubing material consists essentially of a polymerselected from the group consisting of polyamide resins,polytetrafluoroethylene resins, and mixtures thereof.
 19. The method ofclaim 18 wherein said polymeric tubing has a wall thickness less thanabout 0.005 inch.
 20. A method for manufacturing a metal tube having aninterior surface resistant to interaction with materials to be conveyedtherethrough, the method comprising:positioning a length of non-reactivetubing having a predetermined outer diameter within an inner diameter ofmetal tubing of essentially corresponding length having an unsealed sideseam, wherein said inner diameter of said unsealed metal tube is atleast 30% greater than said outer diameter of said non-reactive tubing;forming a suitable sealed side seam in said outer metal tube whilemaintaining said non-reactive tubing at a position opposed to said sideseam during said formation step; and reducing said outer metal tubing toan outer diameter essentially equal to said predetermined outer diameterof said non-reactive interior tubing.
 21. The method of claim 20 whereinsaid suitable side seam is a lock seam.
 22. The method of claim 20wherein said non-reactive tubing is a flexible polymeric material whichis essentially non-reactive when brought into contact with materialswhich are interactive with metal surfaces and wherein said side seamformation step comprises:welding said side seam while maintaining saidnon-reactive tubing at a location within said metal tubing opposed tosaid side seam.